Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing the new design.

The rectangular broken-line perimeter in the FIGURE showing a display screen or portion thereof are for environmental purposes only and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, substantially as shown and described. 